This invention relates elements for water cooled nuclear reactors and nuclear reactors which include the fuel elements. More particularly, the present invention relates to fuel elements having claddings made of a ceramic composite
Cladding for water cooled reactor fuel elements has traditionally been made of ductile metal alloys such as zircaloy, an alloy of zirconium, or stainless steel, in order to provide the strength, corrosion resistance, pressure tightness to hold fission gases, and low neutron capture cross section. These characteristics are required to transfer heat reliably and safely from the contained nuclear fuel to the water coolant. The present metal alloy materials are satisfactory during normal operations. However, during accident situations involving overheating of the fuel, metal claddings have several serious drawbacks
First, metal claddings lose much of their strength above 1000.degree. F., and since accidents sometimes yield temperatures of 3000.degree. F. and higher, they fail to contain the radioactive gases, and indeed the uranium oxide fuel, during such accidents. Such was the case during the accident at the Three Mile Island Nuclear Plant in 1979.
Secondly, they react exothermically with hot water above about 1500.degree. F., thus adding additional heat to the decay heat which continues to be generated by the nuclear fuel. This additional heat exacerbates the severity and duration of the accident, as it did at Three Mile Island.
Thirdly, metal claddings produce hydrogen gas when reacting with water or steam above 1500.degree. F. Hydrogen is a flammable gas which can react violently with air or oxygen creating additional damage and heat, and sometimes threatening the integrity of the reactor containment building. Nuclear plants often contain extensive hydrogen mitigation systems in an effort to prevent or mitigate the reaction of hydrogen with air and the threat to the integrity of the containment building.
Silicon carbide has been used as a cladding in gas cooled nuclear reactors. In water cooled reactors, however, it is desirable to provide a cladding that is inert in high temperature H.sub.2 O Silicon carbide is not inert in high temperature H.sub.2 O but rather reacts with H.sub.2 O to produce hydrogen gas. The production of hydrogen gas poses the risk of a hydrogen explosion within the reactor containment building. Also, in high temperature H.sub.2 O, silicon carbide reacts exothermically which further produces undesirable heat in a reactor under accident conditions.